1. Field of the Invention
The present invention relates to a method of and an apparatus for obtaining a halftone image from an original image having gradation with an image recording apparatus such as a scanner.
2. Description of the Related Art
When it is intended to produce an halftone image from an original image having gradation with an image recording apparatus, moire may occur due to interference between a regular pattern and a halftone dot, which is well-known. Japanese Patent Laying Open Gazette No. 2-136255 (1990), for example, discloses a method of reducing such moire.
According to this method, a process is started with preparation of a plurality of conversion functions which are employed in converting a level of either a screen pattern or an image signal. After the level conversion, imaginary solid-pixel numbers to be produced are calculated in response to the corresponding converted signals, where "solid-pixel numbers" means the numbers of recorded pixels which are exposed to light. Each of these imaginary solid-pixel numbers are compared with a predetermined reference solid-pixel number to thereby select one conversion function which gives the imaginary solid-pixel number most approximate to the reference solid-pixel number. Then, one of the screen pattern and the image signal is converted again through the selected conversion function. The converted one is compared with the non-converted one to thereby produce a halftone-dot exposing signal (when the screen pattern is converted, the converted screen pattern is compared with the image signal, for example). According to this method, a halftone-dot block having a prescribed size is divided into a plurality of blocks, and each of these divided halftone-dot blocks is processed individually.
According to the above-described conventional method, a large number of conversion functions (about thirty kinds, preferably) must be prepared in order to obtain a halftone image of a good quality and with less moire. Either the screen pattern or the image signal is converted through each of these conversion functions, thereafter the corresponding imaginary solid-pixel numbers are computed. Furthermore, comparisons are required between each of the computed imaginary solid-pixel numbers and the reference solid-pixel number. Accordingly, a hardware of a significantly large scale is required in order to carry out this series of processings, that is a problem in the conventional method.